Lead-Zinc Processing Solution

Lead-zinc ores are the classic test of selective flotation. Galena (PbS) and sphalerite (ZnS) sit side by side in the same ore, both float readily with sulphide collectors, and both often carry payable silver. The job is not to float the sulphides, that is easy, but to float them one at a time so the smelter receives two clean concentrates instead of one penalized bulk product. Xinhai builds the flowsheet around that differential separation.

Why differential flotation

A bulk lead-zinc concentrate is almost worthless: lead and zinc smelters pay for their own metal and penalize the other as a contaminant. The economic flowsheet therefore separates the two. The standard route floats galena first because it is naturally more floatable, while sphalerite is depressed with zinc sulphate (often with sodium cyanide or a cyanide-free SO2 scheme where cyanide is restricted). Once the lead concentrate is removed, the depressed sphalerite is reactivated with copper sulphate and floated at elevated pH, typically 10.5-11.5 with lime, to reject pyrite. Getting the depressant and activator balance right is the whole game.

The recommended flowsheet

Crushing, grinding and classification

Ore is crushed with a jaw crusher and cone crusher, then ground in a wet ball mill closed with a hydrocyclone cluster to 60-80% passing 75 micron. Liberation is critical for fine-grained or intergrown Pb-Zn ores; under-grinding leaves middlings that smear grade across both concentrates, while over-grinding makes slimes that hurt selectivity.

Sequential lead then zinc flotation

The conditioned pulp enters the lead circuit, a rougher-scavenger-cleaner train of mechanical flotation cells, dosed with a selective collector and a sphalerite depressant. The lead concentrate is cleaned two or three times to a 50-65% Pb grade. The lead tailing then goes to the zinc circuit, where copper sulphate activates the sphalerite and a fresh collector floats it to a 48-58% Zn concentrate. The full flotation equipment range covers rougher, scavenger and cleaner duties, and a packaged flotation plant can be configured for the two-circuit layout. Silver, if present, generally follows the lead concentrate and adds payable value.

Dewatering

Both concentrates are thickened in a thickener and filtered to a low-moisture cake, because concentrate moisture is freight paid on water over long export hauls. Tailings are thickened and stored with clarified water returned to the mill.

Design choices that drive results

• Reagent scheme: the depressant-activator pair (zinc sulphate, cyanide or SO2, copper sulphate) decides how cleanly Pb and Zn separate; it is set from testwork, never copied.
• Grind size: fixed by liberation; the single biggest lever on misplacement between the two concentrates.
• Oxidation: oxidized lead-zinc minerals float poorly and may need sulphidization or a separate route, so the sulphide-to-oxide ratio must be measured first.
• Pyrite and iron: high pyrite raises lime demand and complicates zinc selectivity.

For a worked regional example, see the 1,200 t/d lead-zinc flotation plant in Morocco, and for the selective-separation logic in a related ore, read our copper flotation flowsheet guide. Two lead-zinc ores rarely behave alike, so Xinhai runs the ore test, designs the differential circuit and delivers the complete plant under an EPC+M+O contract. To size a flowsheet for your ore, contact us for an ore test.